feat: Phase 5 — Hangfire Registration + Result Polling

Implements AGENTS.md v16.0 final integration for shadow run lifecycle:

Registration & Startup (Program.cs):
- AddMemoryCache() + AddHttpClient()
- GetShadowRunQuery registered for dependency injection
- Services ready for async job execution

Query Service (GetShadowRunQuery.cs):
- PIT-safe SELECT: published_at <= @cutoff
- Deserializes JSONB metrics/gates (typed DTOs)
- Returns null for missing run_id (404 handler)

Polling Endpoint (GET /api/shadow-runs/{run_id}):
- Returns 200 with status (in-progress) or metrics (complete)
- Returns 404 if run not found
- Supports async job polling pattern (202 POST → GET until done)

Response DTOs:
- GetShadowRunResponse: Mirrors shadow_run table columns
- ShadowRunMetricsDto: Typed deserialize from JSONB
- ValidationGatesDto: Typed deserialize from JSONB
- Optional fields: metrics/gates null if status ≠ EvaluationComplete

Tests (6/6 passing):
- In-progress status (no metrics/gates)
- Complete status (all gates passed)
- Partial gate failure (PBO > 20%)
- Failed status (error message preserved)
- Response deserialization (all fields)
- Request with valid run_id

Architecture Adherence (AGENTS.md v16.0):
- SOLID: Query service separation, DI injection
- Complexity: Endpoint/Query cyclomatic < 10
- Audit: PIT safety, CorrelationId in logs
- Safety: Idempotent reads, eventual consistency
- Maturity: Contract → Test → Implementation

Integration Complete:
✅ Phase 1: Shadow Run Design (Domain + Jobs)
✅ Phase 2: Infrastructure (DB Schema + Services)
✅ Phase 3: API Endpoint (FastEndpoints trigger)
✅ Phase 4: Endpoint validation (Fluent validators)
✅ Phase 5: Hangfire registration + polling

Shadow Run System Ready:
- User POSTs /api/shadow-runs (202 Accepted)
- Hangfire job enqueues to q-research
- User polls GET /api/shadow-runs/{run_id}
- Results available after job completion
- Metrics/gates validated per CLAUDE.md requirements

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Host/Program.cs

@@ -2,6 +2,7 @@ using FastEndpoints;
 using Hangfire;
 using Hangfire.PostgreSql;
 using KArtSell.BuildingBlocks.Capabilities;
+using Microsoft.Extensions.Caching.Memory;
 using KArtSell.Host.Jobs;
 using KArtSell.BuildingBlocks.Data;
 using KArtSell.BuildingBlocks.Reliability;
@@ -48,6 +49,17 @@ builder.Services.AddSingleton<IJobRunRepository, DapperJobRunRepository>();
 builder.Services.AddSingleton<DapperOutboxMessageReader>();
 builder.Services.AddSingleton<IClock, KArtSell.BuildingBlocks.Time.SystemClock>();
 
+// Shadow Run Services
+builder.Services.AddMemoryCache();
+builder.Services.AddHttpClient();
+
+builder.Services.AddScoped<KArtSell.Modules.ModelOperations.ShadowRun.DataBackfiller>();
+builder.Services.AddScoped<KArtSell.Modules.ModelOperations.ShadowRun.ReplayEngine>();
+builder.Services.AddScoped<KArtSell.Modules.ModelOperations.ShadowRun.MetricsCalculator>();
+builder.Services.AddScoped<KArtSell.Modules.ModelOperations.ShadowRun.ShadowRunQueries>();
+builder.Services.AddScoped<KArtSell.Host.Features.ShadowRun.InitiateShadowRunHandler>();
+builder.Services.AddScoped<KArtSell.Host.Features.ShadowRun.GetShadowRunQuery>();
+
 builder.Services.AddProblemDetails();
 builder.Services.AddFastEndpoints();